change a field

Dear,

I want to change a field of a record after the modification of another 
field of the same record or during an insert of a new record.

Suppose the following table

field        | type
----------------------------------
id        | serial
sign        | integer
value        | integer
real_value    | integer

When inserting a new record, giving values to sign and value, the field 
real_value should be the product of the fields sign and value.
When updating an existing record, the value of the field real_value should 
be recalculated.

I tried with plpgsql functions and triggers but with no success. It works 
only when inserting new records and not when updating a field.

What is the best way to solve this problem ?

Kind regards

Patrick Meylemans